Composition for and method of chemically coating aluminum



Patented June 14, 1 949 COMPOSITION FOR AND METHOD OF CHEMICALLY COATINGALUMINUM Frank Palin Spruance, Jr.,

Ambler, and James H.

Thirsk, Wyncotc, Pa., assignors to American Chemical Paint Company,Ambler, Pa., a corporation of Delaware No Drawing. Application February7, 1947, Serial No. 727,270

12 Claims. (Cl. 148-616) This invention relates to the art of coatingaluminum and alloys thereof in which aluminum is the principalingredient. For example, the invention has been successfully applied notonly to pure aluminum but also to a variety of commonly used alloysconsisting largely of aluminum and is especially useful with copperbearing aluminum alloys. Alloys typical of those with which theinvention is especially useful are the following, the designations givenbeing those employed in the 1946 edition of the Aluminum Company ofAmericas publication on Aluminum and Its Alloys which gives thecomposition of each:

The principal objects of the invention are to increase the durability ofaluminum or aluminum alloy objects or surfaces; to produce protective ocoatings on aluminiferous surfaces which are extremely flexible andwhich will remain substantially integrated with the surface even thoughenough tensile stress is applied to the object to cause it to fail; toprovide a coating which affords an unusually high degree of resistanceto corrosion especially under the severe conditions created by humid orsalt-laden atmospheres such as are encountered in the tropics or near orover the sea; to provide a novel solution and a simple process forproducing acoating on aluminiferous metals; to improve the degree ofdurability of the protection afforded by paints, lacquers, etc. whichare applied to thecoating of the present invention; and to attain theforegoing objectives by means which are more simple and more economicalthan any which have been employed hitherto.

Before proceeding with a detailed description of the invention we wishto refer to the fact that, generally speaking, coatings which containmetallic phosphate have not heretofore been entirely satisfactory inpreventing corrosion and acting as a base for the application of thefinal organic finishing coat, under conditions where the surfaces towhich the coatings have been applied have been subjected to abnormalstrains and stresses. It is well known, for example, that metallicphosphate coatings are somewhat brittle in character and tend to chip orbreak oil when Iii subjected to distortion. The present invention isparticularly useful in overcoming the difficulty just referred to.

The invention is based primarily upon the discovery that the treatmentof aluminum or alloys thereof in which aluminum is the principalingredient by means of aqueous acid solutions containing phosphoricacid, chromic acid, a water soluble compound of fluorine and a watersoluble compound of arsenic in a certain well-defined region as to theproportions of the ingredients, leads to the formation of an integral,strongly adherent, smooth and, apparently, amorphous coating which isextremely flexible and has outstanding merit in inhibiting corrosion andimproving paint durabilty even under abnormally severe conditions such,as are encountered in humid or salt-laden atmospheres and where greatstress or strain are imposed on the metal.

The form in which the active ions are introduced seems to make little orno difference as long as these remain in the solution in the correctproportions and the solution has the proper acidity. For instance, thephosphate ion may be introduced as phosphoric acid or as a salt ofphosphoric acid such as monosodium, monopotassium or mono-ammoniumphosphate; the fluoride ion may be introduced as a solution ofhydrofluoric acid, as sodium fluoride, or as potassium acid fluoride;the dichromate ion may be introduced as chromic acid (ClOa), aspotassium, or as sodium chromate or dichromate, and the arsenate ion maybe introduced as arsenic pentoxide or as sodium arsenate or othercompounds of arsenic soluble in the solution. Naturally the amount ofacid which is to be added will depend upon the form in which theessential ions,are added to the solution, as will be further explainedbelow,

As indicated above, the solutions used in carrying out our improvedcoating process are characterized by a content of acid, fluoride ion,phosphate ion, dichromate ion, and arsenate ion which lie within acertain well-defined area. Generally stated, the region ofconcentrations within which our solutions must be maintained forsatisfactory coating is deflnedas follows:

1. The phosphate ion plus the arsenate ion must be at least 2 grams perliter, and, preferably, at least 6 grams per liter, calculated asequivalent 3 P04. At concentrations below 6.0 grams per liter, theoperative concentrations of dichromate ion and fluoride ion becomeextremely critical; below 2.0 grams per liter, it is hardly possible tomaintain the dichromate and fluoride concentrations with suflicientaccuracy to operate the process even upon a small area of metal per unitvolume of bath. At 6.0 grams per liter of P04 (including arsenatecalculated as P04), the range of permissible fluoride and dichromate islarge enough to permit practical operation on a succession of metalparts. However, even at this concentration rather careful control andfrequent restoration of phosphate, arsenate, fluoride, acid and chromateare necessary to maintain the solution in optimum working condition. Agood working concentration of' phosphate calculated as P04 content isbetween 20 and 100 grams per liter. A practical maximum is about 285grams per liter.

2. The ratio of fluoride ion to dichromate ion, by weight, (calculatedas chromic acid, CrOa) must be between 0.135 and 0.405, and, preferably,between 0.18 and 0.36.

Too high a ratio'of fluoride ion to dicromate leads to the formation ofloose coatings or the absence of coatings and the production of asurface which is merely etched, in contrast with the development oftight, adherent, continuous coatings resistant to abrasion and bending,which are formed under optimum conditions.

A ratio of fluoride ion to dichromate ion which is somewhat too lowleads to the development of very thin coatings; at even lower ratios novisible coating action takes place, and the metal remains smooth andbright.

3. The total acidity of the solution must not exceed that correspondingto 3.0 normal acid. For the purposes of this calculation all polybasicacids whose second ionization constants are less than may be consideredto be monobasic. For such acids the second and subsequent ionizablehydrogen atomsmake less than a 1.0% contribution to the hydrogen ioncontent of their aqueous solutions at pH=2.0 or below. The solutionswith which we are dealing have pH's near or below 2.0. Examples of suchacid are phosphoric and arsenic acids.

4. The arsenic present in the solution should be in amounts of 0.20 to3.0 grams per liter calculated as AszOs.

It must be explained that the absolute quantities of acid and of anionswhich may be present under operative conditions are not independent. Theactual hydrogen ion concentration of the solution is a function of thedissociation constants of the acids corresponding to the anions present.

Therefore, in the absence of anions of acids weaker than phosphoric,hydrofluoric or chromic acid, the hydrogen ion content of the solutionfalls with an increase in the quantity of phosphate, fluoride, arsenateand dichromate. It has been found necessary to have in the solution agreater amount of acid, the greater the quantities of these ionspresent.

It is desirable to express the acidity of the operative solutions interms of pH. Unfortunately no accurate means of measuring the pH ofthese solutions has been found. The use of indicators is unreliablebecause the indicators are oxidized by the dichromate ion present. Theelectrical pH meter using the glass electrode is unreliable because ofthe effect of the fluoride ion upon the glass. The hydrogen andquinhydrone 4 electrodes are likewise the oxidizing effect of thedichromate.

The glass electrode, however, seems to give readings which, while theyoften exhibit a curious excursion with time from a low value to a valueas much as a pH unit higher, and then back to a value even lower than atfirst, seem to have some significance, even though they are notunequivocally interpretable.- Since a recheck in a stand ard buffersolution ofthe glass electrode, after a measurement of one of ourcoating solutions, shows very little change, it may be assumed that theelectrode is not permanently damaged by use for the measurement of ourcoating solutions.

With these limitations in mindjthe final, nearly steady reading of pH,by means of a commercial glass electrode pH meter, in our improvedsolutions in correct operating condition, falls in the range 1.6 to 2.2,and for optimum coating conditions, in the range 1.7 to 1.9.

To prepare a solution for operating our improved coating process theremayv be added to water:

1. Sufficient phosphate ion plus arsenate ion in the form of phosphoricacid or any phosphate and arsenic acid, or any arsenate soluble at a pHof about 2, to give a phosphate-arsenate content (as PO4) of at least'2grams per liter, and better, at least 6.0 grams per liter.

2. Materials containing fluoride and hexavalent chromiumin a quantitysufficient to give a ratio of dissolved fluorine to dissolved hexavalentchromium (calculated as FICI'Os) of between 0.135 and 0.405, or,preferably, between 0.18 and 0.36. The optimum generally is near 0.27.

3. Arsenic in the form of a compound soluble in the solution to give acontent of arsenic between 0.2 and 3.0 grams per liter,.calculated asAS205. For this purpose, arsenic pentoxide itself, an arsenate solubleat a DH of about 2, or other compound which will yield dissolved arsenicin the solution, may be employed.

4. An acid, preferably one at least as strong as HF, to give an apparentpH as measured by a commercial glass-electrode pH meter of 1.6 to 2.2or, preferably, from 1.7 to 1.9 as measured by the lowest valueindicated within the first 10 minutes of immersion of the glasselectrode in the solution.

This step is, of course, subject to the limitations stated above. In anycase, provided the phosphate, dichromate, fluoride and arsenic arepresent in the proper quantities, the exact quantity of acid to be usedmay be checked by the appearance of the coating produced during theactual processing of aluminum surfaces. Too low an acidity leads to nocoating or a very thin coating. Too high an acidity leads to loosepowdery coatings; still higher acidity to a strong etch, sometimespreceded by the formation of visible coatings which wash off on removalof the treated part from the bath or on rinsing it with water.

In carrying out our improved process the surfaces to be coated should bemoderately clean. The cleaning, which forms no part of the presentinvention,'may be carried out by conventional methods. For example,grease and dirt may be removed by a mild silicate alkali spray or by theuse of an emulsion of a grease solvent. Heavy oxide films may b removedby acid or caustic soda treatments. The cleaned work, which may be wetor dry, istreated with a solution of proper composition, of which oneexample is the followinapplicable because of Foam No. I

Phosphoric acid, 75% grams 61 Sodium fluoride -do 5 Chromlc acid (CrOs)do-; Arsenic acid do 2 Water to make liter 1 The treatment may beperformed by immersing the surfaces to be coated in the solution, byflowing or spraying the solution upon the work, or by other convenienttechniques in which the solution is allowed to act upon the work; If thesolution is merely applied to the work momentarily after which theadhering film of solution is allowed to act for some time, it isdesirable to use a solution considerably more concentrated than that ofFormula No. I.

The action of the solution may be accelerated by heat. Th solution maybe kept at any temperature from ordinary room temperature to 180 F. ormore. Similar coatings appear to be formed independent of temperaturebut the time for complete coating formation at room temperature may bereduced from about 5 to 10 minutes to 1 to 2 minutes, or even less bysuch a rise in temperature.

Two typical alternative bath formulas are given below by way ofillustration, illustrating a few of the many variations in compositionwhich our improved compositions may take within their operating range.

FORMULA No. II

Maintenance of our solutions in operating condition during theprocessing of a succession of surfaces requires merely that theproportion of dissolved ions and acidity be kept within the prescribedlimits by suitable additions of chemicals. It is to be noted that thecoating operation consumes chemicals as follows:

1. Acid is consumed by attack on the metal. This is accompanied by anevolution of hydrogen during the coating operation.

2. Phosphate, arsenate and fluoride are included in the coating, asevidenced by its analysis.

3. Dichromate is consumed by reduction to trivalent chromium, some ofwhich is included in the coating. some of which remains dissolved in thesolution. a

4. The accumulation of dissolved aluminum in the solution leads,ultimately, to a loss of fluoride as a precipitate of aluminum fluoride.

5. Some trivalent chromium may be precipitated when this accumulates toa suflicient degree as the fluoride and/or phosphate.

These losses, as well as gross loss of solution due to drag-out on thesurface of the work, must be replaced to maintain the bath within itsoperating limits. The precipitates referred to are apparently withouteffect except as they are mechanically objectionable. In any case, theymay be removed without difflculty by decantation or filtration.

After the treatment with our improved solution, as described, the coatedsurfaces can either be rinsed with water and then dried, or be dried"first, followed by a water rinse and a second drying. In the secondinstance the adhering treating solution dries upon the coated surface,and when it is not desired to paint the surface it may be left in anunrinsed condition after it has been dried. However, if paint or otherorganic finish is to be applied to the coated and dried surface, itshould then be thoroughly rinsed with pure water to remove all solublesalts because such salts are likely to cause blistering of the paint orother organic film, especially if the surface is to be subjected orexposed to humid conditions.

We have found that the corrosion resistance imparted to the surface isdistinctly improved by permitting the adhering treating solution to dryupon the coated surface before any rinsing takes place and that suchdried and unrinsed surfaces are satisfactory for many purposes where asubsequent organic finish is not desired but, as stated, where paint isto be applied rinsing is necessary to remove any residue of solublesalts which may be present.

The drying of the coated surfaces with their adhering solution may beaccomplished at ordinary room temperature or at elevated temperatureswhere expedition of the drying process is desirable. If the nitricacid-insoluble type of coating described below is not desired, the timeand temperature of exposure to the drying oven should be limited to thatrequired to remove physical moisture only. Longer heating, especially attemperatures above the boiling point of water, will drive off chemicallybound water and insolubilize the coatings as described below.

Where paint or other organic finish is to be applied to the coatedsurface it may be unnecessary in many instances to go to the trouble ofan initial drying of the adhering residues of treating solution, inwhich event the treated surfaces may be immediately rinsed with purewater to remove the soluble salts and then dried. Such treatment yieldsexcellent results although, as indicated above, the corrosion resistanceof the coating is not quite equal to that of coatings produced bypermitting the adhering treating solution to dry before any rinsingtakes place.

If any small amount of soluble salts should be left on the finally driedsurface they may be rendered much less harmful lithe surface is treated.after coating and drying, with a dilute solution containing free chromicacid. Thus, if there is any possibility that the water used for rinsingis too high in dissolved salts for absolute safety, it has been founddesirable finally to rinse the coated and dried surfaces with a dilutechromic acid solution containing from /2 to 8 ounces of chromic acid pergallons of water, after which the surfaces are again dried. Thistreatment cannot be harmful and may, therefore, be applied as a matterof routine whether or not the water supply is known to be too high insoluble salts.

We should like to note that, if desired, rinsing of the dried residuesof treating solution may in effect be combined with the final chromicacid rinse although to prevent undue contamination of the chromic acidsolution we prefer to rinse first with plain water and then with thedilute chromic acid solution.

The coatings produced by our improved process fluorine, phosphorus,chromium, arsenic, oxygen and hydrogen as their principal constituents.On heating they lose up to 40% of their weight. Physically such heatingseems to produce no obvious change other than a possible darkening ofthe color of the coating. Chemically, however, the coatinngs become muchmore resistant after dehydration by heating. For example, as produced bytreatment with our improved process, but before heating the coatings arefairly readily soluble in 70% nitric acid. After heating they dissolveonly with great difllculty and on long boiling in this acid. Theinertness of the heated coatings is of considerable advantage in theprotection of aluminum which is exposed to corrosive materials andenvironment.

Although, as previously stated, our coating solutions can be preparedfrom a variety of starting substances, possibly the simplest, cheapest,and most easily available combination of chemicals from which to preparethem is an alkali fluoride. phosphoric acid, chromic acid, and arsenicpentoxide.

Although exact maximum and minimum amounts of fluoride and dichromate tobe used in our improved solutions are diflicult to specify. aside fromthe FZC1O3 ratio, it has been found, generally, that:

1. The fluoride ion content should lie between 0.9 and 12.5 grams perliter, and preferably between 2.0 and 6.0 grams per liter.

2. The dichromate ion content should correspond to a total CrOa contentof between 3.75 and 60 grams per liter, and preferably between 6.0 and20 grams per liter.

A good balance between economy in draggedout chemical, ease of control,and good results in coating is obtained in the preferred rangespecifled.

Since the essential ingredients of our coating solution are fluorideion, phosphate ion, dichromate ion, arsenate ion and hydrogen ion, ithas been found desirable in making up and replenishing the solution touse concentrated admixtures which need only to be added to water or toacidifled water to produce operative solutions of the propercomposition. Such admixtures have very obvious advantages.

For making a fresh solution, the concentrated admixture may contain, forexample, compounds of fluorine, of phosphorus (as orthophosphate), ofarsenic (as pentavalent arsenic) and of hexavalent chromium, all in aform soluble in water at pH about 2. The composition should contain theconstituents in the following proportions:

' Bat Fluorine, by weight i"'1 Chromium calculated as ClOs 2.47 to 7.40Phosphate calculated as P04 1.8 to 67.5 Arsenic as AS205 .2 to 3 Theabove admixtures may or may not be compounded to include free acid. Theinclusion of acid is desirable from the standpoint of ease in preparingthe actual coating solutions, since nothing but water and theconcentrated admixture is necessary. However, strong acid solutionscontaining fluoride and chromate are corrosive and, therefore, dangerousto handle. For this reason, the acid is often omitted from theconcentrated make-up composition.

Concentrated compositions may be made up as solutions, slurries orsolids. For ease in shipping and handling, solid admixtures areparticularly 8 desirable. To get usable coating solutions, these needonly to be added to water, acidulated to the proper degree.

A preferred embodiment of our invention as regards make-up material forour improved coating solution, embodying only easily obtainablechemicals is as follows:

Foams No. IV

When the total material of Formula No. IV is dissolved in to 300 gallonsof water, from 3.0 to 4.3 gallons of 20 B. hydrochloric acid will berequired to yield a solution in optimum 0perative condition. The actualamount of hydrochloric acid will depend on the amount of water used.

No purpose would be served in multiplying the number of suchformulasgiven.

It must be noted that the consumption of the various ingredients duringthe coating of a succession of surfaces is not in the same proportion inwhich these constituents exist in the solution. In general, the relativerates of consumption of the ingradients are generally about as follows,by weight:

Fluorine, grams 1.0 Chromium. as ClOa 0.7 to 1.4 Acid, (gram equivalentsof replaceable hydrogen) 0.06 to 0.14 Phosphate, as P04, grams 0.5 to1.0 Arsenate. as A5205, grams 0.006 to 0.012

Since the tolerance of our improved coating solution for variations fromthe optimum ratio of ingredients is reasonably large, it is possible forlimited periods to effect replenishment of the solution with any of theconcentrated make-up materials previously described. However, it hasbeen found that if the same solution is to be used for coating a largearea of metal, the relative rate of consumption of the ingredients issufllciently different from their initial relative concentrations sothat concentrated admixtures designed for making up the originalsolution are not capable of indefinitely maintaining the proper ratiosof constituents in the working bath. Therefore, we have found itdesirable to prepare admixtures having a ratio of ingredients morenearly like that in which they are consumed. For this purpose, compoundsof fluorine, phosphorus (as orthophosphate), of arsenic (as arsenicpentoxide) and of hexavalent chromium, all soluble in water at about pH2, are admixed in the ratio shown above.

If the replenishing material is also to contain acid, which otherwisemust be added separately, the ratio grams fluorine: gram equivalents ofacid, (replaceable hydrogen) should be from 1:0.05 to 1:010.

Examples of solid and liquid replenishing admixtures are as follows:

FORMULA No. V

FORMULA N0. VI

(Liquid) Pounds HF 52.8 CrOa 28.9 H3PO4 (75%) 17.9 A5205 0.

Replenishing the bath with the liquid replenishing material aloneusually results in maintaining it in good operating condition for a longtime. However, it is not wise to prepare the liquid replenishingmaterial too far in advance as it is not entirely stable, although itsrate of decomposition is quite low and for this reason the solidreplenishing material is usually preferred.

It will be seen from the foregoing that in the concentrated admixtures,for each part by weight of fluorine. the quantity of chromium,calculated as CrOa, will range from a minimum of 0.7 parts to a maximumof 7.40 parts, that the quantity of phosphorus calculated as P04. willrange from a minimum of 0.5 to a maximum of 67.5 parts for each part byweight of fluorine and finally that the quantity of pentavalent arsenicas AS205 will range from a minimum of 0.006 parts to a maximum of 3.0parts for each part by weight of fluorine. It will also be seen thatwhen the admixture is to be used for preparing a fresh solution, theminimum amount of chromium should be 2.47 parts by weight for each partof fluorine. that the minimum amount of phosphorus should be 1.8 partsfor each part of fluorine, and that the minimum amount of arsenic shouldbe 0.2 part for each part of fluorine. Still further it will be seenthat when the concentrated admixture is to be employed in replenishing aused solution the maximum amount of chromium for each part of fluorineshould be 1.4 parts, the maximum amount of phosphorus for each part offluorine should be 1.0 part and,

the maximum amount of arsenic should be 0.012 part for each part byweight of fluorine.

Attention is called to the following copending applications of one ofthe present applicants; namely, Spruance, Jr.;

S 3242: l 11mg Date 614. 795 Sent. 6,1045

Grams per liter Fluorine 0.9 to 12.5

Chromic acid (CrOa) 3.75 to 60 Phosphate plus arsenate (calculated asP04) 6.0 to 258.0 Arsenate (AS205) 0.2 t 3.0

the ratio of fluoride ion to dichromate, expressed as FzCrOa, beingbetween 0.135 and 0.405; and and the pH of the solution being betweenabout 1.6 and 2.2, as measured by the lowest value indicated by a glasselectrode pH meter within the first ten minutes of immersion of theelectrode in the solution.

2. The solution of claim 1 in which the ions are present in amountsstoichiometrically equivalent to:

Grams per liter Fluorine 2.0 to 6.0

Chromic acid (CrOa) 6.0 to 20.0 Phosphate plus arsenate (calculated asP04) 20.0 to 100.0

Arsenate (AS205) 0.2 to 3.0

the ratio of fluoride to dichromate, expressed as F: CrOa, being between0.1.8 and 0.36, and in which the pH of the solution, measured asdescribed, lies between 1.7 and 1.9.

3. In a process for coating surfaces of aluminum and alloys thereof inwhich aluminum is the principal ingredient, the step which consists insubjecting the surface to the action of an acid aqueous solution theessential active coatingproducing ingredients of which are fluoride ion,dichromate ion, phosphate ion, arsenate ion and hydrogen ion, the ionsbeing present in amounts stoichiometrically equivalent to Grams perliter Fluorine g 0.9 to 12.5 Chromic acid (C103) 3.75 to 60.0 Phosphateplus arsenate (calculated as P04) 6.0 to 285.0 Arsenate (AS205) 0.2 to3.0

the ratio of fluoride ion to dichromate, expressed as F:Cr0a, beingbetween 0.135 and 0.405; and the pH of the solution being between about1.6 and 2.2, as measured by the lowest value indicated by a glasselectrode pH meter within the first ten minutes of immersion of theelectrode in the solution.

4. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1 and then allowing adhering solutionto dry upon the surface.

5. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1, allowing adhering solution to dryupon the surface, rinsing the dried surface, and again drying it.

6. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1, allowing adhering solution to dryupon the surface. rinsing the Surface in a solution of chromic acidcontaining from A.; to 8 ounces of chromic acid per gallons; and againdrying it.

7. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1; rinsing the surface and drying it.

8. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1; rinsing the surface in a solutionof chromic acid containing 11 from to 8 ounces of chromic acid per 100gallons; and drying it.

9. A process for coating aluminum and alloys thereof in which aluminumis the principal ingredient consisting in subjecting the surface to asolution such as defined in claim 1 and then at least partiallydehydrating the coating formed by heating it.

10. For use in a process for coating aluminiferous metal in which themetal is treated with an aqueous acid solution as defined in claim 1; aconcentrated admixture comprising compounds of fluorine, of phosphorusas orthophosphate, pentavalent arsenic, and of hexavalent chmmium, allin a form soluble in water at pH2, in which admixture, for each part byweight of fluorine there are: (1) a minimum of 0.7 parts and a maximumof 7.40 parts of chromium calculated as C103. (2) a minimum of 0.5 and amaximum of 67.5 parts of phosphorus, calculated as P04, and (3) aminimum of 0.006 parts and a maximum of 3.0 parts of arsenic, calculatedas AS205; the minimum amount of chromium being 2.47 parts when theadmixture is used for preparing a fresh solution and the maximum amountbeing 1.4 parts when the admixture is used for replenishing a usedsolution; the minimum amount of phosphorus being 1.8 parts when theadmixture is used for preparing a fresh solution and the maximum amountbeing 1.0 part when the admixture is used for replenishing a usedsolution; and the minimum amount of arsenic as AS205 being 0.2 part whenthe admixture is used for preparing a fresh solution and the maximumamount being 0.012 part when the admixture is used for replenishing aused solution.

11. An admixture for preparing a coating solution for use in coatingaluminum and alloys thereof in which aluminum is the principalingradient, the essential active coating-producing ingredients of whichadmixture consist of compounds of fluorine yielding fluoride ions insolution, of phosphorus as orthophosphate, of pentavalent arsenic, andof hexavalent chromium, all in forms soluble in water at pH2 and inwhich for each part by weight of fluorine there are from 2.47 to 7.40parts of chromium, calculated as CrOa; from 1.8 to 67.5 parts ofphosphorus, calculated as P04; and from 0.2 to 3.0 parts of arsenic,calculated as AS205.

12. An admixture solution for use in coating aluminum and alloysgredient, the essential active coating-producing ingredients of whichadmixture consist of compounds of fluorine yielding fluoride ions insolution, of phosphorus as orthophosphate, of penta valent arsenic, andof hexavalent chromium, all in a form soluble in water at pH2 and inwhich for each part by weight of fluoride there are from 0.7 to 1.4parts of chromium, calculated as CrOa; 0.5 to 1.0 part of phosphorus,calculated as P04; and 0.006 to 0.012 AS205.

FRANK PALI N SPRUANCE, JR.

JAMES H. THIRSK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

